Low-dose lysergic acid diethylamide alters subjective visual perception but not visual contrast surround suppression.

Theoretical models and empirical evidence suggest that psychedelics alter contextual influences on perception. To test this, we examined contrast surround suppression, a visual illusion in which a grating appears lower in contrast when embedded in a high-contrast surround. While psilocybin has been reported to enhance this and related illusions at moderate-to-high doses, it is unknown whether low-dose Lysergic Acid Diethylamide (LSD) produces similar effects. In a randomized, double-blind, within-subjects crossover study, N = 30 healthy participants received placebo, 10, and 20 μg of LSD (base-equivalent). In each session, participants performed a contrast discrimination task and rated subjective drug effects. LSD did not measurably influence contrast surround suppression. However, participants reported subjective visual changes, including blurrier, more saturated, and more dynamical visual perception. These subjective alterations did not correlate with individual differences in contrast-surround suppression. Our findings indicate a dissociation between subjective experience and early visual contextual processing, suggesting that subjective visual effects of low-dose LSD may arise through mechanisms distinct from those mediating contrast surround suppression. Future research should test whether higher doses of LSD affect contrast surround suppression.